This invention relates to a printing device for varying a printing energy applied by a printing hammer to a type in accordance with a character to be printed.
In a printing device using a printing wheel having types at the corresponding end portions of its respective spokes, a type having a wider contact area with respect to a printing ribbon, such as a type "W", is struck by a printing hammer on a paper sheet with a great printing energy so that printing may be effected with substantially equal printing energy per unit contact area for all the types, i.e., printing may be achieved through the printing ribbon onto the paper sheet with a substantially equal ink concentration for all types. In order for the printing energy per unit contact area to be kept completely constant for all types, a complex circuit arrangement is required and because ordinary printing energy requires setting various preset values.
In order to vary the printing energy in general, it is necessary to control the turn-on time or the conduction level of a current which is supplied to a solenoid for driving the printing hammer. That is, a greater turn-on time or conduction current level is employed for a greater printing energy.
The aforementioned printing device for varying the printing energy level in accordance with the shape of the type or character has the following tasks to be solved.
FIG. 1 shows a positional relation among a printing hammer 1, types 2 formed at the top end portions of spokes on the printing wheel, printing ribbon 3, printing paper sheet 4 and platen 5. With D1, D2 and D3 representing distances of the forward end of printing hammer 1 with respect to type 2, printing ribbon 3 and platen 5, respectively, when the hammer driving solenoid is energized, printing hammer 1 is driven in a direction of an arrow in FIG. 1 to cause type 2 to be struck against platen 5 through printing ribbon 2 and printing paper sheet 4.
The characteristic curves of FIG. 2 at this time show a relation between a time and a distance of printing hammer 1 from its reference position and, as shown in FIG. 2, they greatly vary depending upon the level of a printing energy applied to printing hammer 1, for example, upon a turn-on time TH of an energization current supplied to the hammer driving solenoid. The characteristic curve A in FIG. 2 shows a printing hammer movement characteristic when the turn-on time TH of the hammer solenoid is set to a time TH.sub.0 corresponding to a smaller printing energy. The characteristic curve B shows a printing hammer movement characteristic when the turn-on time of the hammer solenoid is set to a time TH.sub.1 corresponding to a greater printing energy. As appreciated from a comparison between the characteristic curves A and B, since the speed of movement of printing hammer 1 is lower for a smaller energy level, a time (t.sub.2 -t.sub.0) taken from the start of the movement of the hammer at time t.sub.0 to its return to an original position at time t.sub.2 after the striking of the hammer against platen 5 at time t.sub.1 is markedly greater than a corresponding time (t.sub.4 -t.sub.0) taken in the same way when the printing energy is at a higher level.
The time at which, subsequent to driving the printing hammer at a given printing cycle and printing one character on the paper sheet on the platen, the printing wheel starts its rotation at the next printing cycle is necessary to be set to a time equal to, or later than, the time at which the distance of the hammer 1 from its reference position becomes shorter than a distance D1. Thus, the time at which at the next printing cycle the printing wheel starts its movement is set to about the time t.sub.2 at which the printing hammer 1 returns from the start of movement of the printing hammer at time t.sub.0 to the original position after it has been moved along the characteristic curve A.
However, if the movement start times were all set at an equal value in spite of a difference between the curves A and B, that is, a difference in an energy with which the type is printed, there was a waste wait time (t.sub.2 -t.sub.4), presenting the problem of lowering the printing speed as a whole.